This invention relates to color liquid crystal displays and, in particular, to a technique for backlighting a color liquid crystal display.
Liquid crystal displays (LCDs) are commonly used in battery operated equipment, such as cell phones, personal digital assistants (PDAs), and laptop computers, and are replacing bulky CRTs as television screens and computer monitors. Presently, drawbacks of such LCDs include limited brightness, low efficiency, and limited viewing angle. LCDs can be monochrome or color and can be transmissive or reflective. The present invention deals with a color, transmissive LCD that requires backlighting, where the backlighting contains red, green, and blue components.
FIG. 1 is a cross-sectional view of a small portion of a prior art color, transmissive LCD. There are other types of color, transmissive LCD structures. The structure of FIG. 1 will be used to identify certain disadvantages of prior art LCDs that are avoided by the present invention.
In FIG. 1, an LCD 10 includes a white light source 12 to provide backlighting for the upper LCD layers. A common source for white light is a fluorescent bulb. Another white light source is a combination of red, green, and blue light emitting diodes (LEDs) whose combined light forms white light. Other white light sources are known. These white light sources must provide homogeneous light to the back surface of the display.
A popular technique for providing such a homogeneous white light is to optically couple the fluorescent bulb or LEDs to a light guide, such as by optically coupling the light source to one or more edges of a sheet of clear plastic. The sheet has deformities that bend the light approximately normal to the top surface of the sheet so that light is emitted from the surface. Examples of such deformities include ridges in the bottom surface, reflective particles embedded into the plastic sheet, or a roughening of the top or bottom surface of the sheet. The deformities cause a quasi-uniform plane of light to be emitted out the front surface of the light guide. A non-specular reflector may be placed behind the back surface of the light guide to improve brightness and uniformity.
It is also common to not use any light guide, wherein a light source positioned behind the display is provided with appropriate diffusers to uniformly distribute the light across the display.
A polarizing filter 14 linearly polarizes the white light. The polarized white light is then transmitted to a transparent thin film transistor (TFT) array 16 having one transistor for each pixel. TFT arrays are extremely well known and need not be further described.
The light output from the TFT array 16 is then filtered by an RGB pixel filter 18. The RGB pixel filter 18 may be comprised of a red filter layer, a green filter layer, and a blue filter layer. The layers may be deposited as thin films. As an example, the red filter contains an array of red light filter areas coinciding with the red pixel areas of the display. The remaining portions of the red filter are clear to allow other light to pass. Accordingly, the RGB pixel filter 18 provides a filter for each R, G, and B pixel in the display.
Above the RGB pixel filter 18 is a liquid crystal layer 20, and above liquid crystal layer 20 is a transparent conductive layer 22 connected to ground. The absence of an electrical field across a pixel area of the liquid crystal layer 20 causes light passing through that pixel area to have its polarization rotated orthogonal to the incoming polarization. An electrical field across a pixel area of the liquid crystal layer 20 causes the liquid crystals to align and not affect the polarity of light. Selectively energizing the transistors controls the localized electric fields across the liquid crystal layer 20. Both normally open (white) and normally closed (black) shutters are used in different displays.
A polarizing filter 24 only passes polarized light orthogonal to the light output from the polarizing filter 14. Therefore, the polarizing filter 24 only passes light that has been polarized by a non-energized pixel area in the liquid crystal layer 20 and absorbs all light that passes through the energized portions of the liquid crystal layer 20. The magnitudes of the electric fields across the liquid crystal layer 20 control the brightness of the individual R, G, and B components to create any color. In this manner, any color image may be presented to the viewer by selectively energizing the various transistors in the TFT array 16.
Other types of LCDs substitute a passive conductor grid for the TFT array 16, where energizing a particular row conductor and column conductor energizes a pixel area of the liquid crystal layer at the crosspoint.
The RGB pixel filter 18 inherently filters two-thirds of all light reaching it, since each filter only allows one of the three primary colors to pass. This is a significant factor in the generally poor efficiency of the prior art LCDs. The overall transmissivity of the LCD layers above the white light source 12 is on the order of 4-10%. What is needed is a technique for increasing the brightness of an LCD output without requiring additional energy for the white light source.
FIG. 2 illustrates another prior art color LCD. The layer labeled LCD layers 28 may include all the layers in FIG. 1 except for the RGB pixel filter 18 or may be any other layers for implementing an LCD. FIG. 2 does not use a white light source but instead sequentially energizes red, green, and blue light sources 30, such as red, green, and blue LEDs. A light guide 32 typically receives the RGB light along one or more of its edges and bends the light toward the LCD layers 28 using any one of a number of well known techniques. Sequentially energizing the RGB light sources requires synchronization with the energization of the TFT array. Additionally, to avoid any perceivable flicker, the R, G, and B light sources must each be energized at a frequency of at least 180 Hz to accommodate all three colors sequentially at 60 frames per second. The switching speed may need to be even faster to account for motion artifacts such as those caused by the viewer moving his head while viewing the display. Problems with slow switching speed of the shutter (LC+TFT) and motion artifacts will likely keep this approach impractical for at least several more years.
In one embodiment, a color, transmissive LCD uses red, green, and blue LEDs as the light source. The red LED is optically coupled to a first edge of a rectangular light guide; the green LED is optically coupled to a second edge of the light guide; and the blue LED is optically coupled to a third edge of the light guide.
Deformities in the light guide direct light out of the front surface of the light guide. A first set of deformities is arranged to only direct the red light out of the light guide in the red pixel areas of the display. A second set of deformities is arranged to only direct the green light out of the light guide in the green pixel areas of the display. And, a third set of deformities is arranged to only direct the blue light out of the light guide in the blue pixel areas of the display. In one embodiment, these deformities are ridges having angled surfaces generally facing the direction of the incident light to be directed out of the light guide. In such an embodiment, there are three sets of ridges, each set having angled surfaces orthogonal to the other sets. The R, G, and B LEDs are constantly on and there is no color filtering.
In one embodiment, the LCD has red pixels arranged in a column, green pixels arranged in an adjacent parallel column, and blue pixels arranged in a column adjacent to the green pixels. The pattern repeats. For this type of display, the deformities associated with each of the colors in the light guide are arranged in strips coinciding with the columns of pixels for the particular color reflected by the deformities.
In another embodiment, a blue LED is optically coupled to one or more edges of a light guide, and phosphor strips are placed on a surface of the light guide coinciding with the red and green pixel columns. Deformities below the red and green phosphor strips reflect blue light to the backs of the phosphor strips. The phosphor strips coinciding with the red pixel columns generate red light when irradiated with the blue light from the blue LED. The phosphor strips coinciding with the green pixel columns generate green light when irradiated with the blue light. In areas coinciding with the blue display pixels, no phosphor strips are used, but instead, deformities in the light guide are used to leak out the blue light.
If an ultraviolet light LED is used as the light source, phosphor strips for the blue pixel columns are used, which generate blue light when irradiated with ultraviolet light.
Since the inventive backlighting techniques allow the light source(s) to be on 100% of the time, unlike the technique shown in FIG. 2, and no RGB filter pixel is required, unlike the technique shown in FIG. 1, the LCDs of this invention overcome the various drawbacks previously described.